1. Field of the Invention
The present invention relates to a switching power source device, and, more particularly, to a switching power source device which performs a switching operation according to the status of a load.
2. Description of the Related Art
A switching power source device performs a switching operation to store energy in an inductor, and supplies the stored energy to a load. Examples of a switching power source device which performs a switching operation according to the status of a load are disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2002-171760 and Unexamined Japanese Patent Application KOKAI Publication No. 2002-171761.
The switching power source device disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2002-171760 performs a switching operation intermittently when the load becomes light. The switching power source device disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2002-171761 has a low frequency operation mode to lower the switching frequency when the load becomes light.
FIG. 7 is a structural diagram showing one example of a conventional switching power source device having a low frequency operation mode.
This switching power source device sets a reset-set flip-flop (hereinafter simply called “flip-flop”) 2 which is synchronous with the oscillation frequency of an oscillator (OSC) 1.
When the flip-flop 2 is set, a switching element 3 is switched on.
The ON action of the switching element 3 causes a switching current to flow to the primary winding of a transformer 4 from a power source 18, thus storing energy in the transformer 4.
A resistor 5 generates a voltage corresponding to the switching current.
A comparator 6 resets the flip-flop 2 when the voltage generated by the resistor 5 exceeds a target voltage Vm.
The target voltage Vm is a reference voltage ES1, generated by a reference voltage source 7, divided by the resistance of a resistor 8 and the resistance of a light receiving element 9 of a photocoupler.
When the flip-flop 2 is reset, the switching element 3 is switched off.
When the switching element 3 is switched off, the energy stored in the transformer 4 is supplied to a load by a diode 10 and a capacitor 11.
An output-voltage detection circuit 12 detects the difference between an output voltage and a predetermined value. A light emitting element 13 of the photocoupler emits light according to the difference.
The emission of the light emitting element 13 changes the resistance of the light receiving element 9, thus changing the target voltage Vm.
When the load increases through such control, the target voltage Vm becomes higher and the ON duration of the switching element 3 becomes longer.
The target voltage Vm is supplied to the oscillator 1 via a switch 15. When the target voltage Vm becomes lower, the oscillation frequency of the oscillator 1 becomes lower.
A comparator 14 compares the target voltage Vm with a reference value Vb.
When the load becomes lighter and the target voltage Vm becomes lower, the output of the comparator 14 goes to a low level (hereinafter referred to as “L”) from a high level (hereinafter referred to as “H”), switching on the switch 15 which is switched on in response to an “L” signal.
When the switch 15 is switched on, the target voltage Vm is input to the oscillator 1. Accordingly, the oscillation frequency of the oscillator 1 is lowered, so that the normal mode is changed to the low frequency operation mode.
To prevent the mode from returning to the normal mode due to noise or the like immediately after the mode is changed to the low frequency operation mode, the output of the comparator 14 may be given a hysteresis characteristic by changing the reference value Vb in two levels.
FIG. 8 is a structural diagram showing one example of a switching power source device which performs an intermittent switching operation, and uses the same reference numerals as used in FIG. 7 to denote like or same components.
In the switching power source device, the output terminal of the flip-flop 2 is connected to one input terminal of an AND gate 20.
The output terminal of the AND gate 20 is connected to the switching element 3.
The other input terminal of the AND gate 20 is connected to the output terminal of the comparator 14, and is supplied with an “H” signal when the switching power source device is activated.
After activation, the flip-flop 2 is set in synchronism with the oscillation of the oscillator 1.
When the flip-flop 2 is set, the output signal of the AND gate 20 becomes “H”. The switching element 3 is switched on in response to the “H” signal, causing the switching current to flow to the primary winding of the transformer 4.
The resistor 5 generates a voltage corresponding to the switching current.
The comparator 6 resets the flip-flop 2 when the voltage generated by the resistor 5 exceeds the target voltage Vm.
The target voltage Vm is the reference voltage ES1, generated by the reference voltage source 7, divided by the resistance of the resistor 8 and the resistance of the light receiving element 9 of the photocoupler.
When the flip-flop 2 is reset, the output signal of the AND gate 20 goes to “L”, switching off the switching element 3.
Accordingly, the energy stored in the transformer 4 is supplied to the load by the diode 10 and the capacitor 11.
The output-voltage detection circuit 12 detects the difference between the output voltage and the predetermined value. The light emitting element 13 of the photocoupler emits light according to the difference.
The emission of the light emitting element 13 changes the resistance of the light receiving element 9, thus changing the target voltage Vm.
When the load increases through such control, the target voltage Vm becomes higher and the ON duration of the switching element 3 becomes longer.
The comparator 14 compares the target voltage Vm with the reference value Vb.
When the load becomes lighter and the target voltage Vm becomes lower, the output of the comparator 14 goes to an “L” from an “H”.
Accordingly, the AND gate 20 outputs an “L” signal regardless of the output signal of the flip-flop 2, stopping the switching of the switching element 3.
That is, the normal mode is switched to a standby mode.
According to the switching power source devices in FIGS. 7 and 8, even with a constant load, when an input voltage Vin from the power source 18 becomes higher, the target voltage Vm becomes lower, whereas the input voltage Vin becomes lower, the target voltage Vm becomes higher.
According to the switching power source devices in FIGS. 7 and 8, the peak value of a switching current i theoretically becomes constant irrespective of whether the input voltage Vin is high or low for the following reason. Because energy ε per single switching is given by=½×L×i2,
output power Po under PWM control of a fixed frequency f in critical mode becomesPo=ε×f. 
In the actual switching circuit, however, a noise preventing filter (not shown) and the comparator 6 suffer a detection delay. When the input voltage Vin is high, the inclination of the switching current becomes sharp, whereas when the input voltage Vin is low, the inclination of the switching current becomes gentle. That is, the detection time varies.
Therefore, the target voltage Vm becomes a value having the detection delay absorbed by a feedback circuit, and becomes low when the input voltage Vin is high, and becomes high when the input voltage Vin is low.
As the reference value Vb for determining whether the mode should go to the low frequency operation mode or the standby mode is constant, a load current Io at the mode transition to the low frequency operation mode or the standby mode becomes large when the input voltage Vin is high, and becomes small when the input voltage Vin is low.
When the mode is switched to the low frequency operation mode or the standby mode with the input voltage Vin being high, therefore, the switching energy is high, which may generate noise. If the switching power source device is so set as to prevent generation of noise, the mode is not switched to the low frequency operation mode or the standby mode when the input voltage Vin is low. In this case, power consumption under a light load cannot be reduced.